In terms of results, Croatia has historically been better. Also the Croatian's knack for always producing talented and entertaining players like Boban, Prosinecki and more recently Modric makes them a likeable team. I'm no expert on Balkan football, but Serbia despite them having good players, have always been a bit uninspiring for me, they never really get you that interested in watching them. Regardless of whose better, i'd much rather watch the likes of Modric, Kranjcar, Rakitic over Stankovic, Krasic and Tosic.